WASD controls the character, space jumps, number keys 1 through 4 switch between weapons, mouse aims and fires, E repairs barricades, I opens the inventory.
Days 2 Die is a survival horror shooting game where you need to fight off a horde of flesh-eating zombies while armed to the teeth with weapons and ammo. Simple enough.
At the start of each day, you can purchase weapons and barricades. What weapons do is obvious, rips through zombies as fast as the RPM will allow, the higher, the faster. Using this you can calculate the effectiveness of each weapon, but ultimately faster weapons are better than slower ones.
When placing barricades, you only have a limited amount to place, but the smartest way is to keep them close together. When you're done with that, you can purchase your weapons and replenish your ammo. When the zombies attack, they come from both sides. As you would expect, headshots do more damage.
Zombies can quickly tear down your barricades if they accumulate, so make sure to stick near them and repair them while gunning down the undead. They can damage them faster than you can repair them, and the zombies also have very high attack power and speed, so avoid melee at all possible times because one screwup will net you nothing but a game over.
Once you have survived enough days on a specific stage, you advance to the next.